The growing penetration of renewable energies in the electrical system is transforming the stability of the grid, forcing a redefinition of regulation and operation paradigms.

Grid Forming emerges as a key solution to ensure the stability of a 100% renewable energy mix , replacing the mechanical inertia of thermal power plants with virtual inertia based on power converters.

Strategic Energy Europe participated in the Open Session , organized by AEPIBAL , where experts Eugenio Domínguez and Xavier Benaides, CEO and CTO of HESSTEC, addressed the technical, regulatory and operational implications of this technology in energy storage and the electrical grid of the future, as a main conclusion they mentioned that this “will change the rules of the game” for the sector.

Grid Forming vs. Grid Following: The Leap Towards Grid Stability

Grid Forming is a technology that allows power converters to generate the voltage and angle of the grid, unlike traditional Grid Following systems , which simply follow the signal from the existing grid.

This difference is essential to ensure stability in networks with high renewable penetration .

“In a Grid Following system, inverters rely on a pre-existing grid to operate, which limits their ability to respond to disturbances. In contrast, with Grid Forming, inverters can generate their own voltage and frequency reference, providing inherent stability to the grid,” explains Benaides.

The impact of this technology on the energy transition is crucial. As large thermal and nuclear power plants are withdrawn from the system, mechanical inertia disappears , increasing the risk of abrupt frequency variations and blackouts.

In this context, Grid Forming allows storage and renewable systems to behave as virtual synchronous generators, providing synthetic inertia and stabilising the grid in real time .

Inertia and its implications

One of the most critical aspects in the operation of an electrical grid is inertia , that is, the ability of a system to withstand sudden changes in frequency. In traditional grids, this function is provided by large synchronous generators with rotating masses. However, in a renewable grid, where electronic inverters predominate, this inertia is lost .

“Grid Forming allows us to emulate the inertial response of conventional generators, providing stability against grid events such as load fluctuations or unexpected disconnections,” said Domínguez.

A key indicator is the Rate of Change of Frequency (ROCOF) , which measures the speed of frequency drop in response to disturbances. The lower the inertia in the network, the greater the frequency drop and the greater the risk of blackout.

Simulations were presented in the session where the use of Grid Forming inverters managed to drastically reduce the frequency drop in disconnection events, even compared to traditional synchronous generators.

A concrete example of the importance of inertia in system stability is the disconnection of the interconnection between Spain and France in July 2021. In that event, Spain lost 2.5 GW of imports instantly , causing an abrupt drop in frequency.

This incident tested the ability of the Spanish electrical system to maintain frequency stability without the inertial support of the rest of Europe .

The interconnection with France acts as a buffer of stability for the Spanish grid, allowing energy to be imported and exported according to demand, while providing an important reserve of mechanical inertia through the synchronous generators connected to the European system. When the connection was interrupted, the Spanish grid immediately lost this inertial support, leading to an abrupt drop in frequency .

“When the interconnection with France was disconnected, Spain lost the inertia that Europe provided, which generated a large oscillation in frequency ,” explained Xavier Benaides , CTO of HESSTEC , during the Open Session on Grid Forming organized by AEPIBAL .

To prevent the system from collapsing, emergency measures had to be activated :

• Partial disconnection of demand in certain areas to reduce the load on the network.
• Decoupling from Portugal , leaving the country to operate on its own.
• Activation of rapid response mechanisms , such as the Active Frequency Response Service (SRAP) , which seeks to restore balance in seconds.

The central problem in this event was the loss of inertia , that is, the capacity of the network to withstand sudden changes in frequency. In conventional electrical systems, this inertia is provided by synchronous generators of large thermal and hydraulic power plants , which operate with heavy rotating masses capable of absorbing frequency impacts. However, Spain has been reducing its dependence on these technologies, opting for an energy mix increasingly based on renewables .

In a future with a 100% renewable system , this type of disconnection could generate even more serious problems if technologies such as Grid Forming are not available , which allow synthetic inertia to be provided from renewable storage and generation systems.

“If in 2021 we had had a completely renewable system without synthetic inertia, the event would have been much more difficult to control and the risk of blackout would have been greater ,” warned Eugenio Domínguez .

The simulation presented during the session showed how a system with Grid Forming inverters could have mitigated the frequency drop , avoiding dangerous oscillations and reducing the need for extreme measures such as demand shedding.

This case leaves a clear lesson: without technologies such as Grid Forming, the stability of electrical systems with high renewable penetration will be at risk . The implementation of these solutions is not only an option, but a necessity to guarantee the security of the electricity supply in the future .

Impact on storage: the challenge of cycling and degradation

Storage plays a crucial role in the adoption of Grid Forming. However, this technology introduces an additional challenge: the constant cycling of batteries .

“Grid Forming inverters operate continuously to stabilise the grid, which means that the batteries will be in a constant state of charge and discharge, affecting their useful life,” mentioned Domínguez.

The presentation showed data on the accelerated degradation of lithium iron phosphate (LFP) batteries when subjected to high-frequency charge and discharge microcycles.

To mitigate this impact, some projects are combining batteries with supercapacitors , such as in Shandong, China , where a 3 MW, 1080 MVA-s hybrid system is designed to support these services without compromising battery life.

Regulation: RFG 2.0 and the new requirements for Grid Forming

One of the most relevant points addressed in the session was the regulation around Grid Forming. While countries such as the United Kingdom and Australia have already implemented regulations and inertia markets , in Europe progress is being made with the RFG 2.0 network code .

“The new European grid code already provides for the mandatory use of Grid Forming in type B, C and D plants, which means that any new connected system must guarantee frequency and voltage stability,” explained Benaides.

This change represents a great opportunity for the storage sector , as it will entail the need for batteries capable of operating under this scheme.

Inertial markets: the UK model

The UK is one of the most advanced in terms of implementation. A specific inertia market has been created there , where Grid Forming storage projects receive long-term contracts to provide stability to the network.

“This model makes it possible to make the investment in Grid Forming profitable, ensuring stable income for storage operators and contributing to the energy transition,” Domínguez points out.

In contrast, in Spain remuneration mechanisms for these services have yet to be defined , although the implementation of RFG 2.0 could accelerate their adoption.

The transition to a 100% renewable electricity system is not only a question of clean generation, but also of ensuring stability and security in the network .

Grid Forming is positioned as the key technology to replace the mechanical inertia lost with the output of thermal power plants , allowing a safe integration of renewable energy.

“Grid forming is not optional if we want a stable and secure electricity system. It is the way to ensure that a 100% renewable grid can operate with the same reliability as traditional systems,” concluded Benaides.